1. Field of the Invention
The present invention relates to switching power supply units and electronic apparatuses including the same.
2. Description of the Related Art
FIG. 6 shows a circuit diagram of a switching power supply unit related to the present invention. In this figure, a switching power supply unit 1 includes a transformer T having an input winding N1, a first output winding N2, a second output winding N3, and a feedback winding N4, a DC power source Vcc and a switching element Q1 connected in series to the input winding N1, a control circuit 2 connected between the feedback winding N4 and a control terminal of the switching element Q1, a first rectifying circuit 3 connected to the first output winding N2, a constant-voltage-control circuit 4 connected to the output terminal of the first rectifying circuit 3, a second rectifying circuit 6 connected to the second output winding N3, and a regulator circuit 8 connected to the output terminal of the second rectifying circuit 6. The first rectifying circuit 3 and the constant-voltage-control circuit 4 define a first output circuit 5. Additionally, the second rectifying circuit 6 defines a second output circuit 7. The output voltage of an output terminal V1 of the first output circuit 5 is a first output voltage v1, the output voltage of an output terminal V2 of the second output circuit 7 is a second output voltage v2, and the output voltage of an output terminal V3 of the regulator circuit 8 is a third output voltage v3.
The first output winding N2 and the second output winding N3 share a portion thereof such that the output voltage of the second output winding N3 is lower than the output voltage of the first output winding N2. The first rectifying circuit 3 includes a diode D1 and a capacitor C1. The second rectifying circuit 6 includes a diode D2 and a capacitor C2. The constant-voltage-control circuit 4 includes a resistor R1, a photo diode PD, and a shunt regulator SR, which are connected in series, a resistor R2, and a resistor R3, which are also connected in series. The junction of the resistor R2 and the resistor R3 is connected to a control terminal of the shunt regulator SR. The control circuit 2 includes a phototransistor PT. The phototransistor PT and a photodiode PD define a photocoupler. The control circuit 2 turns a switching element Q1 on and off with a feedback signal from the constant-voltage-control circuit 4. In other words, with the feedback control provided on the primary side of the transformer, constant-voltage-control is performed over the first output voltage vl. As a result, the first output voltage v1 becomes a control output. On the other hand, the second output voltage v2 is not feedback-controlled to the primary side, and therefore becomes a non-control output. Thus, the voltage v2 is input to the regulator circuit 8 to perform constant-voltage control over the voltage v2, and consequently becomes the third output voltage v3 lower than the second output voltage v2.
Next, FIG. 7 shows a detailed circuit diagram of the regulator circuit 8. Such a circuit is disclosed in Japanese Unexamined Patent Application Publication No. 11-262262. In FIG. 7, the regulator circuit 8 includes a transistor Q2 defining an impedance element connected in series to the output terminal of the second output circuit 7, a Zener diode ZD connected between the base of the transistor Q2 and a ground, a constant current circuit Io connected between the output terminal of the first rectifying circuit 3 defining the first output circuit and the cathode of the Zener diode ZD, and a capacitor C3 connected between the emitter of the transistor Q2 and another ground.
In the regulator circuit 8 having the structure described above, with the use of the transistor Q2, the second output voltage v2 is converted into the third output voltage v3 having a voltage value approximately equal to the Zener voltage value of the Zener diode ZD to output the voltage v3.
The output of the first output circuit 5 is not output via the regulator circuit 8. Since the regulator circuit 8 uses the first output voltage v1, the voltage v1 is passed through the regulator circuit as shown in FIG. 6. The first output voltage v1 is output from the regulator circuit 8, and no processing is performed on the output v1 in the regulator circuit 8.
The constant-current circuit Io is provided to improve a ripple elimination rate. When the ripple elimination rate is not required to be high, a resistor is sometimes used.
In the switching power supply unit 1 shown in FIG. 6, when the first output circuit 5 outputting the first output voltage v1 is unloaded or lightly loaded, the electric power which can be supplied to the regulator circuit 8 as the load of the second output circuit 7 decreases. However, when the regulator circuit 8 is not unloaded or lightly loaded and large electric power is required, the input electric power of the regulator circuit 8 increases. As a result, due to cross-regulation of the transformer T, the second output voltage v2 drops, and thereby the regulator circuit 8 does not function properly. Therefore, the third output voltage v3 also drops. To prevent this problem, the second output voltage v2 as a non-control output is usually set in advance to be high in expectation of some degree of voltage drop. Consequently, even when the second output voltage v2 slightly drops, the regulator circuit 8 functions properly. However, when the second output voltage v2 is set to be higher than necessary with respect to the third output voltage v3, under normal operating conditions, power loss in the regulator circuit 8 increases. Thus, this reduces the efficiency of the switching power supply unit.
To overcome the above-described problems, preferred embodiments of the present invention provide a switching power supply unit having a regulator circuit in which it is unnecessary to set the voltage of a non-control output to be higher than a required value. Preferred embodiments of the invention also provide an electronic apparatus incorporating the novel switching power supply unit.
According to a preferred embodiment of the present invention, a switching power supply unit includes a transformer having a primary winding, a first secondary winding, and a second secondary winding, a first output circuit connected to the first secondary winding and outputting a first output voltage constant-voltage-controlled by feedback control provided on the primary side of the transformer, a second output circuit connected to the second secondary winding to output a second output voltage, and a regulator circuit connected to the second output circuit to perform constant-voltage-control over the second output voltage so as to output as a third output voltage, in which a load current flowing through the first output circuit is adjusted according to the third output voltage such that the second output voltage is maintained at at least a desired value.
In addition, in the switching power supply unit, the second output voltage is lower than the first output voltage, and the regulator circuit further includes a first impedance element connected in series to the second output circuit, a second impedance element connected between an output terminal of the first output circuit and a control terminal of the first impedance element, and a voltage control circuit controlling the impedance of the second impedance element such that the third output voltage is maintained constant.
In this switching power supply unit, the first impedance element is a first transistor and the second impedance element is a second transistor.
In addition, the switching power supply unit may also preferably include an overcurrent protecting circuit which causes the second impedance element to be out of conduction when the difference between the first output voltage and the third output voltage is larger than a desired value.
In addition, the switching power supply unit may also preferably include an overcurrent detecting circuit arranged to detect a drop in the third output voltage when the value of a current flowing through the second impedance element is larger than a desired value.
According to another preferred embodiment of the present invention, an electronic apparatus is provided that includes the switching power supply unit of the above-described preferred embodiment of the present invention.
With the above arrangements, in the switching power supply unit and the electronic apparatus according to various preferred embodiments of the present invention, it is unnecessary to set the voltage of the non-control output higher than required. Thus, the efficiency of the switching power supply unit and the electronic apparatus is greatly improved.
Other features, elements, advantages and characteristics of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.